Jonathan D. Judy

Phosphorus (P) dynamics in the soil subsurface are relatively unstudied, especially in forest soils, and may be important in areas with groundwater‐fed surface water bodies. Here, we use X‐ray absorption near‐edge P spectroscopy (P‐XANES) and X‐ray fluorescence microprobe mapping (µ‐XRF) to examine P speciation and co‐localization with Fe and Al as a function of depth in soils from coniferous and deciduous forests. P‐XANES revealed similar trends in P speciation with depth for both forest types, with Al‐P and Fe‐P being dominant, whereas Ca‐P and organic P were minor constituents. Correlation plots derived from µ‐XRF mapping revealed a strong correlation between P and both Fe and Al at all depths. At depths of up to 378 cm, a range that includes A, E, and B horizons, the relationships between P and Al/Fe were remarkably consistent, consisting of a population of P diffusely associated with Al/Fe, consistent with Fe‐ or Al‐adsorbed P, as well as P linearly correlated with Al/Fe in several discreet groupings, indicative of the presence of specific P compounds. At depths below 378 cm, the relationships between P and Al/Fe changed dramatically, with correlation plotting indicating the disappearance of any diffuse P:Al/Fe relationships. This change occurred concurrently with a decrease in oxalate extractable P, suggesting that the P present in this deep soil horizon may be present in a more stable form that is less available for transport, as well as potentially indicating that P adsorbed to Fe/Al did not readily pass through the overlaying Bw soil horizon.